Virtual Instrumentation Laboratory Manual

EI2356 - Process Control System Laboratory

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INDEX

Sl. No

Exp. Date Name of the Experiment Date of

submission Page No.

Marks Staff Sign

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

www.Vidyarthiplus.com



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EI2356 PROCESS CONTROL SYSTEM LABORATORY L T P C 0 0 3 2

1. Study of interacting and non-interacting systems.

2. Response of different order processes with and without transportation lag.

3. Response of on-off controller.

4. Response of P+I+D controller.

5. Characteristics of control valve with and without positioner.

6. Operation of on-off controlled thermal process.

7. Closed loop response of flow control loop.

8. Closed loop response of level control loop.

9. Closed loop response of temperature control loop.

10. Closed loop response of pressure control loop.

11. Tuning of PID controller.

12. Study of cascade (complex) control system.

P = 45, TOTAL = 45

LIST OF EXPERIMENTS

S.No CYCLE I - Experiments Page No

1. Study of Interacting systems.

2. Study of Non-interacting systems.

3. Characteristics of control valve a. Without positioner.

b. With positioner

4. a. Operation of on-off controlled thermal process.

b. Response of on-off controller

5. Closed loop response of level control loop.

6. Closed loop response of pressure control loop.

S.No CYCLE II - Experiments Page No

7. Response of different order processes with and without transportation

lag.

8. Closed loop response of flow control loop.

9. Closed loop response of temperature control loop.

10. Response of P+I+D controller.

11. Response of cascade control system.

12. Tuning of PID controller.




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Experiment No: Date:

1. STUDY OF INTERACTING SYSTEMS Aim:

To study the operation of the interacting system and find its transfer function.

Components required: Interacting system setup and Stop watch.

Theory: The term interacting is referred as loading. To understand the effect of interaction, consider a

two tank system shown in experimental setup. The second tank is said to load the first tank. The flow

through R1 depends on the difference between h1 and h2. The analysis is started by writing mass

balance on the tank. The balances on tank 1 and 2 are the same. The flow head relationship for tank 1 is q2 = (h1 - h2) / R1

The mass balance equation of tank 1 is q1 – q2 = A1 (dh1 / dt ) --- (1)

The mass balance equation of tank 2 is q2 – q3 = A2 (dh2 / dt ) --- (2) The flow head relationships for the two linear resistances are given by the expressions

q2 = h1 / R1 , q3 = h2 / R2 --- (3)

at steady state, the flow equation is q1s – q2s = 0, q2s – q3s = 0

By solving all the above equations using laplace transform, we get the transfer function

H2(s) / Q1(s) = R2 / ( 1 2s2 +1( 1 + 2 + A1 R2)s +1 )

Where q1 = Inflow to tank 1 in lph.

A1 = the area of tank 1.

h1 = Output variable head of tank1. R1 = Resistance of valve in the outlet tank1.

q2 = Inflow to tank 2 in lph.

A2 = the area of tank 2.

h2 = Output variable head of tank2. R2 = Resistance of valve in the outlet tank2.

q3 - Outflow of tank 2 in lph

1, 2 – time constants of tank 1, 2 respectively

Experimental setup: Interacting Tank




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Specifications:

Precautions: 1. Fully open the inlet valve to tank 1.

2. Partially open the valve between tanks 1&2. 3. Partially open outlet valve from tank2.

4. Keep all other valves closed.

Procedure: 1. Switch on the pump.

2. Set the flow rate of liquid at desired value by adjusting the rotameter and wait till it reaches the

steady state in two tanks. 3. Once the head reaches the steady state, give a small step change of flow rate and observe the

heads h1 & h2 of tanks 1 & 2 till tanks reach another steady state.

4. Plot graph between time & h1, time & h2. 5. Find out time constants using the following relations.

1 = time corresponding to the head of 0.632 (final steady state – initial steady state) from graph 1


6. Calculate the resistance R2 using time constant 2 and area of tank 2.

R2 = 2 / A2 7. Find the overall transfer function, by substituting the above calculated values in following

equation, H2(s) / Q1(s) = R2 / ( 1 2s2 +1( 1 + 2 + A1 R2)s +1 )

Tabulation:

S. No Time (t) sec Height of Tank1 (h1 ) cm Height of Tank2 (h2 ) cm




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Model Calculation:

Result: Thus the response of the interacting system was studied and the transfer function was found.

Inference:

Review questions: 1. What is self regulation?

2. What is interaction factor? 3. What is the significance of interaction factor?

4. Why two interacting capacities have more sluggish response than two equivalent but non-

interacting capacities? 5. Comment about response of interacting capacities?




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Experiment No: Date :

2. STUDY OF NON-INTERACTING SYSTEMS Aim:

To study the operation of the non interacting system and find its transfer function.

Components required: Non interacting system setup and Stop watch.

Theory: Consider the two tank system shown in experimental setup. The outlet flow from tank1

discharges directly into the atmosphere before spilling into tank 2 and it flows through R1, depends

only on h1. The variation in h2 in tank 2 does not affect the transient response occurring in tank 1. This

type of system is referred as non-interacting system. Assume the liquid to be of constant density, the tanks to have uniform cross sectional area and the flow resistances to be linear.

The mass balance equation of tank 1 is

q1 – q2 = A1 (dh1 / dt) --- (1) The mass balance equation of tank 2 is

q2– q3 = A2 (dh2 / dt ) --- (2)

The flow head relationships for the two linear resistances are given by the expressions

q2 = h1 / R1, q3 = h2 / R2 --- (3) Combining the equations (1) and (3) we get the transfer function for tank 1 is

Q2(s) / Q1(s) = 1 / ( 1s+1) --- (4)

Where Q2 = q2 – q2s, Q1= q1 – q1s, 1 = R1A1

Combining the equations (2) and (4) we get the transfer function for tank 1 is

H2(s) / Q2(s) = R2 / ( 2s+1)

Where H2 = h2 – h2s, 2 = R2A2 The overall transfer function of non interacting system is

H2(s) / Q1(s) = R2 / ( 2s+1) ( 1s+1) Where, q1 = Inflow to tank 1 in lph. A1, A2= the area of tank 1 & 2.

h1, h2= Output variable head of tank1 & 2.

R1, R2 = Resistance of valve in the outlet tank1 & 2.

q2 = Inflow to tank 2 in lph. q3 - Outflow of tank 2 in lph

1, 2 – time constants of tank 1, 2 respectively

Experimental setup: Non-Interacting Tank Specification:

Precautions:

1. Fully open the inlet valve to tank 1.

2. Partially open the valve between tanks 1&2. 3. Partially open outlet valve from tank2

4. Keep all other valve closed.




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Procedure: 1. Switch on the pump.

2. Set the flow rate of liquid at desired value by adjusting the rotameter and wait till it reaches

the steady state in two tanks.

3. Once the head reaches the steady state, give a small step change of flow rate and note down the heads h1 & h2 of tanks 1 & 2 till tanks reach another steady state.

4. Find out time constants using the following relations.



5. Calculate the resistance R2 using time constant 2 and area of tank 2.

R2 = 2 / A2 6. Find the overall transfer function, by substituting the above calculated values in following

equation H2(s) / Q1(s) = R2 / ( 2s+1) ( 1s+1)

Tabulation:

S. No Time (t) sec Height of Tank1 (h1 ) cm Height of Tank2 (h2 )cm

Model Graph: Model Calculation:




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Result: Thus the response of the non interacting system was studied and the transfer function was

found. Inference:

Review questions: 1. What is non self regulating process? Give example. 2. Define time constant.

3. What is transfer lag?

4. What is the difference between interacting and non-interacting system? 5. Give the characteristics of multi capacity system.




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3a. CHARACTERISTICS OF CONTROL VALVE WITHOUT POSITIONER Aim:

To study the actuator characteristics and control valve characteristics of ON – OFF (Quick

opening) and equal % control valve.

Components required: Control valve trainer setup and Air Compressor (0 – 100 psi).

Theory: In most of the industrial process control systems control valve is the final control element.

The control valve consists of two major components, viz. actuator and valve. The actuator is made up

of flexible diaphragm; spring and spring tension adjustments, plate, stem and lock nut, housing. The valve is made up of body, plug, stem, and pressure tight connection.

The function of a control value is to vary the flow of fluid through the value by means of a change

of pressure to the valve top. The relation between the flow through the valve and the valve stem position (or lift) is called the valve characteristic. There are three main types of valve characteristics.

The types of valve characteristics can be defined in terms of the sensitivity of the valve, which is

simply the fraction change in flow to the fractional change in stem position for fixed upstream and

downstream pressures. Mathematically Sensitivity = dm / dx. In terms of valve characteristics, valve can be classified into three types:

1. Linear

2. Increasing sensitivity 3. Decreasing sensitivity.

For the linear type valve characteristics, the sensitivity is constant and the characteristic curve is a

straight line (e.g. linear valve). For increasing sensitivity type, the sensitivity increases with flow. (e.g. Equal percentage or Logarithmic valve ).In practice, the ideal characteristics for linear and equal

percentage valves are only approximated by commercially available valves. These discrepancies

cause no difficulty because the inherent characteristics are changed considerably when the valve is

installed in a line having resistance to flow, a situation that usually prevails in practice. Equal percentage control valve:

Flow changes by a constant percentage of its instantaneous value for each unit of valve lift.

Quick opening control valve: Flow increases rapidly with initial travel reaching near its maximum at a low lift.

Experimental Setup for Control Valve Positioner:

Precautions:

1. Check the inlet pressure to the air regulator.

2. Don’t operate the motor pump without load.

3. Release the air pressure before conducting the experiment.




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Procedure: 1. Open the appropriate hand valve for ON / OFF control valve and close all other hand

valves.

2. Vary the pressure using the knob provided in the air regulator.

3. Note down the pressure in gauge and corresponding stem movement and flow rate. 4. Reduce the output pressure to zero using the air regulator knob.

5. Release the air in the control valve using the hand valve.

6. Select another control valve and repeat the same procedure. 7. Draw the graph between input pressures to the control valve and stem movement.

8. Draw the graph for stem movement vs. flow rate.

Tabulation:

Table 1 ON-OFF control valve

S.

No

Input pressure

( psi )

Stem movement

(mm)

Flow rate

( lph)

Table 2 Equal percentage control valve

S.

No

Input pressure

( psi )

Stem movement

(mm)

Flow rate

( lph)




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Result: Thus the actuator and control valve characteristics were studied.

Inference:

Review questions: 1. Name some electric actuators.

2. List some pneumatic actuators.

3. Why an equal % valve is called so? 4. What are known as inherent characteristics?

5. Explain on/off valve.




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3b. CHARACTERISTICS OF CONTROL VALVE WITH POSITIONER Aim:

To study the actuator characteristics and control valve characteristics.

Components required: Control valve positioner trainer setup, Air Compressor (0 – 100 psi) and Current source (0 –

20 mA)

Theory: The final control element is a device which alters the value of the manipulated variable in

response to the output signal from the automatic control device. In most of the industrial process

control systems control valve is the final control element which uses pneumatic signal. The control valve consists of two major components, viz. actuator and valve. The actuator is used to translate the

output signal of the controller into a position of a member exerting large power. The valve is a device

to adjust the value of the manipulated variable. The actuator must provide an accurate position proportional to the input signal in spite of

various forces acting on the output member. The important forces are Inertia force, Static friction

force and Thrust force. The actuator often requires a positioner when any one or all above forces are

acting on control valve. The positioner may be pneumatic, hydraulic, electrical or any combination. Normally the positioner is inbuilt with control valve which requires pneumatic signal.

The positioner consists of input bellows, a nozzle and amplifying pilot, feedback levers and

spring. When the input pressure increases the input bellows moves to the right and causes the baffle to cover the nozzle. The nozzle back- pressure change is amplified by the pilot and is transmitted to the

diaphragm. The use of the positioner improves the following performance. Hysteresis is reduced and

linearly improved. The speed of response is generally improved. The positioner has two types. Force balance type and force distance type. The positioner may be operated in “BYPASS MODE” (manual

mode) or “AUTO MODE”

Experimental Setup: Specification:




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Precautions:

1. Check the inlet pressure to the air regulator. 2. Release the air pressure before conducting the experiment.

3. Ensure the gauges G3 and G4 are at 20 psi.

Procedure: 1. Switch the power switch to ‘ON’ position.

2. Set the switch in variable position to vary the current from 4 to 20 mA.

3. Select the bypass mode on the positioner and give 20 mA to I / P converter.

4. Increase the stem movement one by one (0 – 12.5 mm), for each movement note down the corresponding current reading.

5. Change the mode to “Auto” on the positioner and give 20 mA to I / P converter.

6. Increase the stem movement one by one (0 – 12.5 mm), for each movement note down the corresponding current reading.

7. Tabulate the values and draw the graph between stem movement and current.

Tabulation:

Input Pressure

G2(psi)

I/P Current

(mA)

Stem movement

(mm)

I/P Output

PressureG5(psi)

Pilot Pressure

G6(psi)




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Result: Thus the control valve characteristic with positioner was studied.

Inference:

Review questions: 1. Why does flow lift characteristics of a control valve change after it is installed in a pipeline?

2. What is cavitation and flashing?

3. What is the purpose of valve positioner? 4. What is drawback of oversized and undersized valve?

5. What components are present in a positioned?




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4a.OPERATION OF ON-OFF CONTROLLED THERMAL PROCESS Aim:

To obtain the response of the ON-OFF controller (two position controller) for a thermal

process.

Components required: Oven, ON – OFF controller with display and Thermocouple.

Theory: The major components of temperature loop are furnace, heater, temperature transmitter, and

controller.

Temperature Transmitter: Temperature transmitter consists of a thermocouple and signal conditioning circuit.

Thermocouple works on “Seebeck effect”, “Peltier effect” and “Thomson effect”. The working

principle of thermocouple is, when two junctions of two dissimilar metals are kept at two different temperatures an emf is produced which is proportional to the temperature difference. This emf is

converted into current using suitable signal conditioning circuit. There are different types of

thermocouple. Here in our process station K type is used.

Controller: An On – Off controller operates on the manipulated variable only when the temperature

crosses the set point. The output has only two states, usually fully on and fully off. One state is used

when the temperature is anywhere above the set point and the other state is used when the temperature is anywhere below the set point. Since the temperature must cross the set point to change the output

state, the process temperature will be continually cycling. Two position control applied to a process

results in a continuous oscillation in the variable to be controlled. This drawback was overcome by a continuous control action which could maintain a continuous balance of the input and output.

Experimental setup: Specifications:

Procedure:

1. Switch on the power supply. 2. Switch on the heater.

3. Select the ON - OFF controller.

4. Note down the time, temperature and controller output. 5. Let the process to reach the steady state.

6. Draw the graph between temperature vs. time and controller output vs. time.

7. View the responses for different set points.




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Tabulation: Set Point: Differential Gap=

Model Graph:

Result: The response of the ON-OFF controller for a thermal process is obtained.

Inference:

Review questions:

1. What do you mean by two position controller?

2. Give some examples for ON-OFF controlled thermal processes. 3. What are the sensors can be used to measure temperature?

4. What are the materials being used in J type thermocouple?

5. Give the range of temperature can be measured using J type thermocouple.

S.NO TIME(Sec) TEMPERATURE (C) CONTROLLER O/P (CO)




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Experiment No: Date :

4b. RESPONSE OF ON-OFF CONTROLLER Aim:

To study the action of ON-OFF controller for a process.

Apparatus required: Vi Microsystems simulation package.

Theory: An On – Off controller operates on the manipulated variable only when the temperature

crosses the set point. The output has only two states, fully on and fully off. One state is used when the

temperature is anywhere above the set point and the other state is used when the temperature is

anywhere below the set point. Since the temperature must cross the set point to change the output state, the process temperature will be continually cycling. Two position control applied to a process

results in a continuous oscillation in the variable to be controlled. This drawback was overcome by a

continuous control action which could maintain a continuous balance of the input and output. Two-position control applied to a process results in a continuous oscillation in the variable to

be controlled. This drawback was overcome by a continuous control action which could maintain a

continuous balance of the i/p and o/p. A mode of control which could accomplish this is known as

proportional, proportional + integral, proportional + integral + derivative control.

Diagram for ON-OFF Controller:

Procedure: 1. Select ON-OFF controller mode.

2. Enter the controller parameter (set point (SP), differential gap (DG)) in settings. 3. Run the program in simulation mode by properly selecting the input (like sine, triangular,

square).

4. View the response for different set points and control parameters. 5. Plot the graph (PV vs. Time and CO vs. Time) for any one tabular column.




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Tabulation:

a. ON-OFF CONTROLLER

SP= DG=

S.

No

Time (t)

sec

Process

Variable

Controller Output

(CO)(%)

b. SP= DG=

S.

No

Time (t)

sec

Process

Variable

Controller Output

(%)

Result: Thus the action of ON-OFF controller for a process was studied.

Inference:

Review questions: 1. What is hysteresis in on-off control? 2. Give the significance of on-off control.

3. What is dead band?

4. Give the applications of on-off controller. 5. What other names does on-off controller has?




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5. CLOSED LOOP RESPONSE OF LEVEL CONTROL LOOP Aim:

To obtain the closed loop response of level control loop with suitable feedback controller.

Components required: Flow level process station set up, Computer and Patch chords.

Theory: The major components of level loop are Process tank, level transmitter, controller, control

valve, I/P converter, Data acquisition system.

Process Tank:

The process tank has one inlet valve and one outlet valve. The level can be controlled either by controlling the inlet flow rate or outlet flow rate or both.. In the level loop set up the control valve

is placed in the inlet path. So the level of the tank will be controlled by controlling the inlet flow rate.

Level Transmitter: Here the capacitive type transducer is used as a level transmitter. The capacitive transmitter

consists of two parallel plates of same area separated by a small distance. The capacitance C = A / d

Where C – Capacitance

- Dielectric constant A – Overlapping area of plates

d – Distance between the plates distance. Here the two parallel plates are fixed and hence the over lapping area and the distance between the

plates are not going to change. When the tank is empty air is acting as the dielectric medium. When

water level rises there is a change in capacitance due to the change in dielectric medium. This change in capacitance is converted into current using suitable signal conditioning circuit.

Controller:

Two position controls applied to a process results in a continuous oscillation in the variable to be controlled. This drawback was overcome by a continuous control action which could maintain a

continuous balance of the i/p and o/p. The different modes of continuous controllers are proportional

(P), integral (I), derivative (D), PI, PD, PID. The tuning parameters for the PID controller are

1. Proportional gain (Kp) 2. Integral gain (KI)

3. Derivative gain (KD)

Based on the nature of processes a particular mode should be selected.

I/P Converter:

The pickup system consists of a voice coil situated in the air gap of permanent magnet. The

converter consists of a nozzle restriction and baffle plate on the beam which is mounted on the low friction fulcrum. The converter converts the force on the voice coil produced by the interaction

between the magnet and current coil into the movement of the baffle plate which closes the nozzle and

so increases the pressure in the connecting pipe. The pneumatic relay consists of a diaphragm, a valve

seat, a needle and a capillary tube. The valve is opened until the pressure equilibrium on both sides of the double membrane is restored. When the valve is opened the supply air flows directly to the outlet

with a pressure proportional to the valve opening.

The relationship between the input current and the output air pressure is linear as the feedback within the relays is 100%and therefore the pneumatic amplification is 1:1

Control valve:

In most of the industrial process control systems control valve is the final control element. The

control valve consists of two major components namely actuator and valve. The actuator is made up of flexible diaphragm, spring and spring tension adjustments, plate, stem and lock nut, housing. The

valve is made up of body, plug, stem and pressure tight connection.

Based on the principle of operation the control valves can be classified into 1. Linear




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2. Equal percentage

3. Quick opening Linear control valve: Flow is directly proportional to valve lift.

Equal percentage control valve: Flow changes by a constant percentage of its instantaneous value for

each unit of valve lift.

Quick opening control valve: Flow increases rapidly with initial travel reaching near its maximum at a low lift. The output pressure of I/P converter is given to the control valve which changes the opening

of the control valve and hence changes the flow rate.

Data acquisition system: This consists of ADC, DAC,I/V,V/I and RS232. This system acquires data from the

process station to the computer and sends the data from the computer to the process loop.

Panel Diagram: Specifications:




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Block Diagram:

Precautions:

1. Check the air inlet pressure to the process station as 50 – 100 psi.

2. Check whether the I/P converter inlet pressure is 20 psi.

3. Let hand valve HV1 and HV3 be fully opened position. 4. Keep by pass valve HV2 partially opened.

5. Keep the outlet valve HV6 slightly opened.

6. Check whether there is any leakage in air and water path. 7. Check whether the water in the reservoir tank is sufficient to fill the process tank.

8. Don’t switch on the motor when control valve is fully closed.

9. Switch off the mains before making the connections.

Procedure: 1. Give connections between the process and controller (Computer).

2. Switch on the power supply. 3. Switch on the motor.

4. Select a suitable controller.

5. Tune the controller parameters.

6. Observe the response. After reaching the steady state give disturbance by suddenly opening and closing the outlet valve HV6.

7. Let the level of the process tank to reach the steady state.

8. View the response for different set points. 9. Plot the transmitter characteristics, controller characteristics and closed loop response.

Tabulation:

Table 1: Transmitter Characteristics Table 2: I/P Converter

Controller

output (%)

( in % )

Controller output

( mA)

I/P converter

(psi)

Level in

(cm)

Level transmitter

output (mA)




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Table:3 Closed Loop Response of Temperature Control Process

SP = KP =

Time in Sec Process Variable (cm)

In cm

Controller Output (%)

in percentage

Result: Thus the closed loop response of level control loop was obtained.

Inference:

Review questions: 1. Write the limitation of on/off control?

2. Define offset.

3. Define proportional band. 4. What is the maximum level that can be controlled in the given process?

5. Name some direct and indirect level measurement device.




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6. CLOSED LOOP RESPONSE OF PRESSURE CONTROL LOOP Aim: To obtain the closed loop response of pressure control loop with suitable feedback controller.

Components required: Pressure process stations set up, computer and patch chords.

Theory: The major components of Pressure loop are Process tank, Pressure transmitter, controller,

control valve, I/P converter, Data acquisition system and plant.

Pressure Transmitter:

Pressure transmitter consists of a piezoelectric transducer and signal conditioning circuit. The piezoelectric transducer which works on piezoelectric effect has a natural piezoelectric crystal

(quartz) inside it. The piezoelectric effect is when a force is applied along the mechanical axis of

piezoelectric crystal an emf is generated along its electrical axis. This emf is converted into current using suitable signal conditioning circuit.

Controller:

Two position control applied to a process results in a continuous oscillation in the variable

to be controlled. This drawback was overcome by a continuous control action which could maintain a continuous balance of the i/p and o/p. The different modes of continuous controllers are proportional

(P), integral (I), derivative (D), PI, PD, PID. The tuning parameters for the PID controller are

1. Proportional gain (Kp) 2. Integral gain (KI)

3. Derivative gain (KD)


I/P Converter:

The pickup system consists of a voice coil situated in the air gap of permanent magnet. The

converter consists of a nozzle restriction and baffle plate on the beam which is mounted on the low

friction fulcrum. The converter converts the force on the voice coil produced by the interaction between the magnet and current coil into the movement of the baffle plate which closes the nozzle and

so increases the pressure in the connecting pipe. The pneumatic relay consists of a diaphragm, a valve

seat, a needle and a capillary tube. The valve is opened until the pressure equilibrium on both sides of the double membrane is restored. When the valve is opened the supply air flows directly to the outlet

with a pressure proportional to the valve opening.

The relationship between the input current and the output air pressure is linear as the feedback within the relays is 100%and therefore the pneumatic amplification is 1:1

Control valve:


control valve consists of two major components namely actuator and valve. The actuator is made up of flexible diaphragm, spring and spring tension adjustments, plate, stem and lock nut, housing. The

valve is made up of body, plug, stem, pressure tight connection.

Based on the principle of operation the control valves can be classified into 1. Linear

2. Equal percentage

3. Quick opening.

Linear control valve: Flow is directly proportional to valve lift. Equal percentage control valve: Flow changes by a constant percentage of its instantaneous value for

each unit of valve lift.

Quick opening control valve: Flow increases rapidly with initial travel reaching near its maximum at a low lift.

The output pressure of I/P converter is given to the control valve which changes the opening of the

control valve and hence changes the flow rate.




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Data acquisition system: This consists of ADC, DAC,I/V,V/I and RS232. This system acquires data

from the process station to the computer and sends the data from the computer to the process loop.

Block Diagram:

Precautions: 1. Check the air inlet pressure to the process station as 100 – 150 psi. 2. Check whether the I/P converter inlet pressure is 20 psi.

3. Let hand valves HV2, HV4 and HV6 be fully closed position.

4. Keep the hand valves HV1 and HV3 connected tank 1 slightly opened.

5. Keep the outlet valve HV5 of tank 1 slightly opened. 6. Check whether there is any leakage in air path.

7. Switch off the mains before making the connections.




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Procedure: 1. Give connections between the process and controller (Computer). 2. Switch on the power supply.


4. Tune the controller parameters.

5. Observe the response. After reaching the steady state give disturbance by suddenly opening and closing the hand valve HV5.




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6. Let the process to reach the steady state.

7. View the response for different set points. 8. Plot the transmitter characteristics, controller characteristics and closed loop response.

Tabulation:

Table 1: Transmitter Characteristics Table 2: I/P Converter

Controller

output (%)

( in % )

Controller

output (mA)

( mA)

I/P converter

(psi)

Table 3: Closed Loop Response Of Pressure Control Loop

SP = KP = KI=

Time in Sec Process Variable

in psi

Controller Output

in percentage

Result: Thus the closed loop response of pressure control loop was obtained.

Inference:

Review questions: 1. What is meant by differential gap?

2. Name the measurement variable and manipulated variable in pressure control loop.

3. What type of final control element is used here? 4. Which control mode is suitable for pressure process? Justify.

5. What is the nature of pressure process?

Pressure

(psi)

Pressure transmitter

output (mA)




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7. RESPONSE OF DIFFERENT ORDER PROCESSES WITH AND

WITHOUT TRANSPORTATION LAG Aim:

To find the response of the first order system, second order system with and without

transportation lag for step signal and compare the result with Matlab response.

Components Required: First order & second order system setup, MATLAB Tool box and computer

Theory: Any physical system having the following transfer function represented below is called as

first order system.

Y(s) / X(s) = 1 / s + 1

The parameter is called time constant. The transient and steady state response of the first order system can easily obtained to any forcing function like impulse, step, ramp and parabolic.The

response to the different inputs can be expressed in terms of mathematical expression. Most of the responses contain exponential part. The examples first order systems are, built in thermometer, level

tank, electrical RC circuit and etc.

Sometimes the system may have transportation lag. A phenomenon that is often present in flow systems is transportation lag. This is also known as dead time and distance velocity lag. The

transportation lag is quite common in the chemical process industries where a fluid is transported

through a pipe. The present of a transportation lag in a control system can make it much more difficult

to control. In general, such lags should be avoided if possible by placing equipment close together. They can seldom be entirely eliminated.

The transportation lag is quite different from the other transfer function like first order,

second order. The transportation lag can be approximated by different ways. One approach is Taylor series.

e - s

= 1 / (1 + s + 2 s

2/2 +

3 s

3 / 3! + …… )

This can also be approximated to e - s

= 1 / (1 + s)

This can also be approximated using Pade approximation as: e - s

= (1 - s/2) / (1 + s/2)

Experimental Setup for finding the response of First order process:

Specification:




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Block Diagram for finding the response of Second order process:

Procedure: For simulation:

1. Open a new file.

2. Enter the parameters for the first order, second order transfer function. 3. Obtain the response the system for different inputs (like step, impulse, and ramp).

4. Plot the response for different time constants.

5. Save the file and use Simulink toolbox and simulate this program.

Procedure for Experimental Setup: 1. Switch on the pump.

2. Set the flow rate of liquid at desired flow rate by adjusting the rotameter and wait till it reaches the steady state in two tanks.

3. Plot the graph between time and height.

Tabulation:

First Order System (With Delay) First Order System (Without Delay)

S. No Time (t) sec C(t)





29

Second Order System (With Delay) Second Order System (Without Delay)

Result: Thus the response of the first order system, second order system with and without

transportation lag for step signal were obtained and the response is compared with MATLAB simulation.

Inference:

Review questions: 1. What is dead time?

2. What is order of a process? 3. What is the physical meaning of order of the system?

4. Differentiate between system delay and transportation delay.

5. What is the effect of dead time in a process?






30


8. CLOSED LOOP RESPONSE OF FLOW CONTROL LOOP Aim: To obtain the closed loop response of flow control loop with suitable feedback controller.

Components required: Flow process station set up, Computer and Patch chords.

Theory: The major components of flow loop are Orifice plate, Differential pressure transmitter (DPT),

controller, control valve, I/P converter, Data acquisition system, plant.

Orifice Plate:

The working principle of orifice plate is “vena contracta’ effect. When a restriction is introduced in the pipe line the velocity of the fluid through the restriction increases and the pressure

decreases. The relationship between the pressure drop and the rate of flow is

Q = K p; where Q = volume flow rate K = a constant for a pipe and liquid type

p= pressure drop across the restriction. The different types of orifice plates are

1. Concentric

2. Eccentric

3. Segmental

4. Quadrant edge

DPT:

Differential pressure transmitter is a device which converts differential pressure into current.

The differential pressure generated by the orifice plate is given to two sides of parallel plate capacitive transducer made up of thin diaphragms separated by a small distance. When the pressures applied, it

changes the spacing between the diaphragms (d) which changes the capacitance since the equation of

capacitance

is C = A / d

Where C – Capacitance

- Dielectric constant A – Overlapping area of plates

d – Distance between the plates

This change in capacitance is converted into current using suitable signal conditioning circuit.

Controller: Two position controls applied to a process results in a continuous oscillation in the variable to


continuous balance of the I/p and o/p. The different modes of continuous controllers are proportional (P), integral (I), derivative (D), PI, PD, PID. The tuning parameters for the PID controller are

1. Proportional gain (Kp)

2. Integral gain (KI) 3. Derivative gain (KD)


I/P Converter:

The pickup system consists of a voice coil situated in the air gap of permanent magnet. The converter consists of a nozzle restriction and baffle plate on the beam which is mounted on the

low friction fulcrum. The converter converts the force on the voice coil produced by the interaction

between the magnet and current coil into the movement of the baffle plate which closes the nozzle and so increases the pressure in the connecting pipe. The pneumatic relay consists of a diaphragm, a valve

seat, a needle and a capillary tube.

The valve is opened until the pressure equilibrium on both sides of the double membrane is

restored. When the valve is opened the supply air flows directly to the outlet with a pressure proportional to the valve opening.The relationship between the input current and the output air




31

pressure is linear as the feedback within the relays is 100%and therefore the pneumatic amplification

is 1:1

Control valve:


control valve consists of two major components, 1. Actuator 2.Valve. The actuator is made up of

flexible diaphragm, spring and spring tension adjustments, plate, stem and lock nut, housing. The valve is made up of body, plug, stem, pressure tight connection.Based on the principle of operation

the control valves can be classified into Linear, Equal percentage and Quick opening.

Linear control valve: Flow is directly proportional to valve lift. Equal percentage control valve: Flow changes by a constant percentage of its instantaneous value

for each unit of valve lift.

Quick opening control valve: Flow increases rapidly with initial travel reaching near its maximum at a low lift. The output pressure of I/P converter is given to the control valve which changes the

opening of the control valve and hence changes the flow rate.

Data acquisition system:

This consists of ADC, DAC,I/V,V/I and RS232. This system acquires data from the process station to the computer and sends the data from the computer to the process loop.

Block diagram:




32

Panel Diagram: Specification:




33

Precautions: 1. Check the air inlet pressure to the process station as 50 – 100 psi.

2. Check whether I/P converter inlet pressure is 20 psi.

3. Let hand valve HV1 and HV3 be fully opened position.

4. Keep by pass valve HV2 partially opened. 5. Check whether there is any leakage in air and water path.

6. Check whether the water in the reservoir tank is sufficient to fill the process tank.

7. Don’t switch on the motor when control valve is fully closed. 8. Switch off the mains before making the connections.

Procedure: 1. Give connections between the process and controller (Computer). 2. Switch on the power supply and the motor.


4. Tune the controller parameters. 5. Observe the response. After reaching the steady state give disturbance by suddenly opening

and closing the hand valve HV2.

6. Let the process to reach the steady state.

7. View the response for different set points and plot the response. 8. Plot the transmitter characteristics, controller characteristics and closed loop response.

Tabulation:

Table 1: Transmitter Characteristics Table 2: Control Valve Characteristics

Controller output

( % )

Controller output

( mA)

Output

(psi)

TABLE 3: Closed Loop Response

PI controller SP= KP= KI=

Time in Sec Controller Output

in percentage

Process Variable

(lph)

Flow(lph) Flow transmitter

output (mA)




34

Result: Thus the closed loop response of flow control loop was obtained.

Inference:

Review questions: 1. Explain the function of pneumatic transmission line.

2. Why differential control cannot be used alone?

3. What is FCE? 4. How offset can be minimized?

5. Why derivative control is not recommended for noisy process?




35


9. CLOSED LOOP RESPONSE OF TEMPERATURE CONTROL LOOP Aim: To obtain the closed loop response of temperature control loop with suitable feedback

controller.

Components required: Temperature process station set up, Computer and Patch chords.

Theory: The major components of temperature loop are furnace, heater, temperature transmitter,

controller, SCR, Data acquisition system.

Temperature Transmitter: Temperature transmitter consists of a thermocouple and signal conditioning circuit. Thermocouple works on “Seebeck effect”, “Peltier effect” and “Thomson

effect”. The working principle of thermocouple is, when two junctions of two dissimilar metals are

kept at two different temperatures an emf is produced which is proportional to the temperature difference. This emf is converted into current using suitable signal conditioning circuit. There are

different types of thermocouple. Here in our process station K type is used.

Controller: Two position controls applied to a process results in a continuous oscillation in the

variable to be controlled. This drawback was overcome by a continuous control action which could maintain a continuous balance of the I/p and o/p. The different modes of continuous controllers are

proportional (P), integral (I), derivative (D), PI, PD, PID. The tuning parameters for the PID controller

are Proportional gain (Kp), Integral gain (KI), Derivative gain (KD) Based on the nature of processes a particular mode should be selected.

SCR: The SCR is a four layer PNPN device with three terminals, viz. the anode, the cathode and the

gate. The SCR is supplied by 230V. When the gate voltage is varied from 0 to 5V it changes the firing angle which in turn varies the output voltage in the range of 0 – 230 V. The output voltage of SCR

controls the heater current and hence the temperature of furnace.

Data acquisition system: This consists of ADC, DAC, I/V, V/I and RS232. This system acquires

data from the process station to the computer and sends the data from the computer to the process loop.




36

Panel Diagram: Specification:




37

Block diagram:

Procedure:

1. Give connections between the process and controller (Computer). 2. Switch on the power supply and the heater.

3. Select a suitable controller and tune the controller parameters.

4. Observe the response. After reaching the steady state give disturbance by changing the blower speed.

5. Let the process reach the steady state.

6. View the response for different set points.

7. Plot the transmitter characteristics, controller characteristics and closed loop response.

Tabulation:

Table 1: Transmitter Characteristics Table 2: Final control element (SCR)

Characteristics

Controller

output ( % )

Controller

output (mA)

SCR

output (V)

Volts

TABLE 3: Closed Loop Response

SP = KP = KI = KD=

Time in Sec Process Variable

in C

Controller Output

in percentage

Temperature

in C

Temperature transmitter

output (mA)




38

Result: Thus the closed loop response of temperature control loop was obtained.

Inference:

Review Questions: 1. What are the major components of process control system? 2. What is ZOH?

3. What final control element is used in temperature control loop?

4. What control action is preferred for slow process? 5. What are the sensors can be used to measure temperature?




39


10. RESPONSE OF P +I+D CONTROLLER Aim:

To study the action of proportional+ integral+ derivative (PID) controller for a Second order

process.

Apparatus required: Analog PID controller trainer, CRO and Function generator.

Theory: Two-position control applied to a process results in a continuous oscillation in the variable to


continuous balance of the i/p and o/p. A mode of control which could accomplish this is known as proportional, proportional + integral, proportional + integral + derivative control. The proportional

controller produces an output signal that is proportional to the error e. This action may be expressed

as p = Kc e+ ps ; where p = output signal from controller,

Kc = gain or sensitivity

e = error = set point – measured valve,

ps = constant The term proportional band is commonly used among process control in place of the term

gain. Proportional band ( pb) is defined as the error ( expressed as a percentage of the range of

measured variable) required to move the valve from fully open to fully closed. The relation between proportional band ( pb) in percentage and Kc is Kc = 100 / [ pb ( % )]

The proportional – Integral control mode is described by the relation

p = Kc e + Kc / τI ∫ e dt + ps, where τI = integral time The proportional – Integral - Derivative control mode is given by the expression

p = Kc e + Kc τD de/dt + Kc /τI ∫ e dt + ps




40

Panel Diagram:

Procedure: 1. Give the connections in the front panel to obtain the response for different controllers. 2. Set the process fast/slow switch (SW4) in fast position.

3. Keep the set value pot to zero and apply a square wave signal of to Vpp at around 50 Hz.

P Controller 5. Keep the proportional band knob at 20%.

6. Repeat all the above steps with the percentage proportional band 50% and 40%.

7. Observe the response and find the peak overshoot (Mp), Rise time (tr), Peak time (tp),

Damping ratio and settling time (ts) and also tabulate the readings

PI Controller

8. Adjust the proportional band control until the system settles with 2 to 3 overshoots.

9. Connect the integral section. 10. Slowly reduce the integral action time until the deviation falls to zero.

11. Observe the response and calculate the proportional band (PB), Integral time, peak

overshoot (Mp), Rise time (tr), settling time (ts) and also tabulate the readings.

PID Controller 12. Set the process fast/slow switch (SW4) and controller fast/slow switch (SW3) in fast

position.

13. Apply a square wave signal of to 2Vpp at around 50 Hz. 14. Now patch I and I’ and adjust the integral time until the steady state deviation is zero.

15. Now note down the number of overshoots before the system settles.

16. Now connect D and D’ and slowly increase the derivative time and note down the effect of this system responses.

17. Observe the response and calculate the proportional band (PB), Integral time, peak

overshoot (Mp), Rise time (tr), settling time (ts) and also tabulate the readings.




41

Tabular column:

P Controller

Sl.No %Proportional

Band(PB)

Peak

overshoot(Mp),

Peak time

(tp)

Damping

Ratio

settling

time(ts)

PI Controller

Sl.No Proportional

Band(PB)

Integral

Time (Ti)

Peak

overshoot(Mp),

Rise time

(tr)

settling

time(ts)

PID Controller

Sl.No %Proportional

Band(PB)

Integral

time(Ti)

Rise time(tr) Peak

time(tp)

Settling

time(ts)

Peak

overshoot(Mp)

Result: Thus the time response of closed loop second order process with proportional control, PI

control and PID control was studied.

Inference:

Review Questions: 1. Design an electric PI controller for kp=5 and TR=10 sec.

2. Design a P controller with PB=50%. 3. Why is integral action (only I) recommended for zero or low order process?

4. What will be the controller response, if the error is constant?

5. What is reset windup?




42


11. RESPONSE OF CASCADE CONTROL SYSTEM Aim: To study the characteristics of cascade controller for level and flow process.

Components required: 1. Cascade control setup.

2. Computer.

3. Patch chords.

Theory: Two controllers are used in the cascade control system. One is the primary controller which is

in the primary loop also called as master controller. Another one is called secondary controller or slave controller which is connected in the secondary loop. The output of one controller can be used to

manipulate the set point of another.

The flow is sensed with the orifice meter .The water flow (secondary loop) to the process tank is controlled by pneumatic control valve. To maintain, the level (primary loop) under cascade control

logic. The control cubicle houses process indicator/ microprocessor controller, output indicator, power

supply for transmitters, control switch, etc.


Procedure: 1. Fill the tank with water; open the air pressure to 20 psi by regulator. 2. Switch on the water circulation pump. Keep the rotameter valve full open.

3. Select the LEVEL + FLOW CASCADE controller in the system.

4. Keep the output indicator in the auto mode. 5. Enter the controller parameters and obtain the closed loop response.

6. Exit from the software by pressing ESC KEY.

7. Draw the graph between level vs time, flow vs time and CO vs time.




43

Tabulation: SP = Flow: KP = Ki =

Level: KP = Ki = Kd =

Time in

Sec

Process Variable (Primary

Loop-Level) in cm

Process Variable

(Secondary Loop-

Flow) in lph

Controller

Output

in percentage

Result: Thus the characteristic of cascade controller for level and flow process was obtained.

Inference:

Review Questions: 1. How many measurement variable and manipulated variable are present in cascade control

loop? 2. What is multi loop control?

3. Write the advantages and disadvantages of cascade loop.

4. What is procedure to tune cascade controller?

5. What is multi loop control?




44


12. TUNING OF PID CONTROLLER Aim :

To obtain the controller parameter for the given process, using Ziegler–Nichols tuning and

cohen coon method.

Components required: MATLAB package.

Theory: The selection of a controller type (P, PI, PID) and its parameters (Kc, τI, τD ) is intimately

related to the model of the process to be controlled. The adjustment of the controller parameters to

achieve satisfactory control is called tuning. The controller parameters are accurately identified for proper tuning and control of plant or system. To meet out the performance specifications of the

system in the varying condition of operating point and environmental changes, the tuning of controller

plays vital role. Ziegler – Nichols tuning:

The Ziegler – Nichols tuning rules are based on closed loop tuning method. Here the

controller remains in the loop as an active controller in automatic mode. The tuning rules are as

follows 1. After the process reaches steady state at the normal level of operation, remove the integral and

derivative modes of the controller, leaving only proportional control. On some PID controllers,

this requires that the integral time ( τI) be set to its maximum value and the derivative time (τD) to its minimum valve.

2. Select a value of the proportional gain ( Kc), disturb the system, and observe the transient

response. If the response decays, select a higher value of Kc and again observe the response of the system. Continue increasing the gain in small steps until the response first exhibits a

sustained oscillation. The value of gain and the period of oscillation that corresponding to the

sustained oscillation are the ultimate gain ( Kcu) and the ultimate period ( Pu).

3. From the values of Kcu and Pu found in the previous steps, use the Ziegler – Nichols tuning rules to determine controller settings( Kc, τI τD )

Cohen - Coon Method:

The most popular empirical tuning method is the process reaction curve method developed by Cohen and Coon. The Cohen-Coon method is classified as an 'offline' method for tuning, meaning

that a step change can be introduced to the input once it is at steady-state. Then the output can be

measured based on the time constant and the time delay and this response can be used to evaluate the initial control parameters. Consider the closed loop system of fig2 which has been “opened’ by

disconnecting the controller from the final control element. Introduce a step change of magnitude A in

the variable C which actuates the final control element. In the case of a valve, c is the stem position.

Record the value of the Output with respect tp time. The curve ym(t) is called the process reaction curve. Between y and c the following transfer function is obtained

GPRC(s) =ym(s)/c(s): ym(s)/c(s) =Gf(s)Gp(s)Gm(s)

The last equation shows that the process reactionn curve is affected not only by the dynamics of the main process but also by the dynamics of the measuring sensor and final control element. The

response of most processing units to an input change had a sigmoidal shape, which can be adequately

approximated by the response of a first-order system with dead time

GPRC(s) = K e-tds

/ s + 1

which has three parameters: static gain K, dead time td, and time constant . From the approximated response, it is easy to estimate the values of three parameters.

K=output (at steady state)/input (at steady state)

K=B/A, =B/S Where S is the slope of the sigmoidal response at the point of inflection

td=time elapsed until the system responded




45

Advantages:

1. Used for systems with time delay. 2. Quicker closed loop response time.

Disadvantages:

1. Can only be used for first order models including large process delays.

2. Offline method

3. Approximations for the Kc, Ti, and Td values might not be entirely accurate for different

systems.

Diagram:

Fig 1: Zeigler - Nichols Method

Fig 2: Cohen Coon Method – “opened” control loop Process reaction curve:

Procedure:

1. Open a new m-file in MATLAB. 2. Enter the transfer function of the given system and obtain it’s bode plot.

3. Note down the values of ultimate gain and period of oscillation.

4. Open a new simulink window. 5. Draw the corresponding block diagram with the above calculated values of P, PI, PID

controllers using Z -N and C-C formulae and obtain the responses on the scope.

Tabulation:

Ziegler – Nichols tuning Method:

Type of control Gc(s) Kc / Kp τI KI τD KD

Proportional (P) Kc 0.5Ku - - - - - -

Proportional Integral

(PI) Kc( 1 + 1 / τ s) 0.45Ku

Pu /

1.2 - - -

Proportional Integral

Derivative (PID) Kc ( 1 + 1 / τI s +τDs ) 0.6Ku Pu / 2

Pu /

8




46

Cohen - Coon Method:

Type of

control Gc(s) Kc τI τD

(P) Kc (1/K) (τ / td) (1 + (td/3τ)) - -

(PI) Kc( 1 + 1 / τ s) (1/K) (τ / td)

(0.9 + (td/12τ))

((td) (30+ 3 (td/ τ)))/

( 9+ 20 (td/ τ)) -

(PID) Kc ( 1 + 1 / τI s +τDs ) (1/K) (τ / td) (1.34 +

(td/12τ))

((td) (32+ 6 (td/ τ)))/

( 13+ 8 (td/ τ))

((td) (4)) /

( 11+ 2 (td/ τ))

Type of

control Gc(s) Kc τI KI τD KD

(P) Kc - - - -

(PI) Kc( 1 + 1 / τ s) - -

(PID) Kc ( 1 + 1 / τI s +τDs )

Result: Thus the controller parameter was obtained using Ziegler – Nichols tuning and cohen coon

method and the response for different controller modes are compared using MATLAB package.

Inference:

Review Questions: 1. Give some tuning procedures.

2. What is tuning?

3. What is load variable? 4. Define Servo and Regulation operation.

5. What is meant by two degree of freedom controller?



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Virtual Instrumentation Laboratory Manual